1. Field of the Invention
The present invention relates to a wave coil spring which is installed in, for example, a clutch system of an automatic transmission which is an automotive part.
2. Description of the Related Art
Wave coil springs are made by working a strip of spring material into a wave-like shape and winding the wave-shaped spring material in a spiral fashion and are in general widely used as a return spring for a hydraulic piston incorporated in a clutch system of an automatic transmission or as a damping spring in various types of equipment. As a wave coil spring of this type, there is a wave coil spring which is disclosed in JP-A-2002-174282.
FIGS. 7A and 7B are diagrams which explains a problem inherent in a conventional wave coil spring. The figure exemplarily shows vertically adjacent nth coil and n+1th coil of a wave coil spring which are developed in a straight-line fashion. FIG. 7A shows a state in which no compressive load is applied, whereas FIG. 7B shows a state in which a compressive load is applied to the spring.
As is shown in FIG. 7A, in the wave coil spring, peak portions 101, 201 and valley portions 102, 202 are formed to occur alternately by working a spring material into a wave-like shape, and peak portions 101 of the nth coil (lower coil) and valley portions 202 of the n+1th coil (upper coil) are in contact with or face each other at apex portions T. Then, when a compressive load is applied to the wave coil spring in an axial direction thereof, by nature, the compressive load P is applied to the apex portions T of the peak portions 101 and the valley portions 202, whereby the peak portions and the valley portions 202 deflect and deform uniformly about the apex portions T.
However, in the event that deviations are produced in contact portions between the nth coil and the n+1th coil due to working error or assembling error, or an external force being applied from other directions than the axial direction, as is shown in FIG. 7B, there has occurred a case where the contact portions between the nth coil and the n+1th coil slide in one direction, whereby the compressive load P is applied to middle slopes of the peak portions 101 and the valley portions 202.
In this case, since a tangential component Ph of the compressive load P applied to each of the contact portions is directed to the same direction, when the tangential component Ph becomes larger than a frictional force applied to each of the contact portions, a circumferential slide (buckling) is produced at each of the contact portions, leading to a risk that the load bearing performance of the wave coil spring is reduced drastically.
In order to prevent the occurrence of the slide (buckling) of the contact portion like this, a wave coil spring disclosed in JP-UM-A-5-67836 adopts a configuration in which a locking mechanism made up of a recessed portion and a raised portion is formed at apex portions of (an contact portion between) a peak portion and a valley portion, so as to prevent the slide (buckling) thereof by virtue of engagement of the recessed portion with the raised portion.
In addition, a wave coil spring of JP-A-2002-27670 adopts a configuration in which a damping member is mounted at apex portions of (an contact portion between) a peak portion and a valley portion, so as to fix the contact portion in place to thereby prevent the slide (buckling) thereof.